Storage batteries are employed in many applications requiring electrical energy to be retained for later use. Most commonly, they are employed in motor vehicles utilizing internal combustion engines. In such applications, energy stored by "charging" the battery during engine operation is later used to power lights, radio, and other electrical apparatus when the engine is stopped. The most severe demand upon the battery of a motor vehicle is generally made by the self-starter motor. Typically, several kilowatts of power are required by the self-starter motor to crank the engine. Failure to satisfactorily accomplish this task, particularly in cold weather, is usually the first indication of battery deterioration or trouble with the charging system. Clearly, a simple measurement that accurately assesses a battery's ability to supply power is of considerable value.
Prior to the publication of U.S. Pat. Nos. 3,873,911 and 3,909,708, a battery's ability to supply power was customarily assessed by means of a load test. A load test subjects a battery to a heavy dc load current having a predetermined value dictated by the battery's rating and temperature. After a prescribed time interval, the battery's terminal voltage under load is observed. The battery is then considered to have "passed" or "failed" the load test according to whether this terminal voltage is greater than, or less than, a particular presecribed value. Although the load test has been widely used for many years to field-test storage batteries, it possesses several serious disadvantages. These include:
1. Currents drawn are very large and therefore require apparatus that is heavy and cumbersome.
2. Because of these large currents, considerable "sparking" can occur at the battery terminals if the test apparatus is connected or disconnected under load conditions. Such "sparking" in the presence of battery gasses can cause an explosion with potentially serious injury to the operator.
3. A load test leaves the battery in a significantly reduced state of charge and therefore les capable of cranking the engine than before the test was performed.
4. Since the battery's terminal voltage drops continouously with time during the load test, the test results are imprecise and greatly dependent upon the skill of the operator.
5. Load test results are not repeatable since the test itself temporarily polarizes the battery. Such test-induced polarization significantly alters the initial conditions of any load tests performed subsequently.
A practical alternative to the common load test is taught in U.S. Pat. Nos. 3,873,911 and 3,909,708. Both of these patents disclose electronic apparatus for accurately assessing a battery's condition by means of small-signal ac measurements of its dynamic conductance. These two patents teach that a battery's dynamic conductance is directly proportional to its dynamic power; the maximum power that the battery can deliver to a load. Dynamic conductance is therefore a direct measure of a battery's electrical condition. Virtually millions of battery measurements performed over the course of thirteen years have fully corroborated these teachings and have proven the validity of this alternative testing method.
In comparison with the load test method of battery appraisal, the dynamic conductance testing method taught in U.S. Pat. Nos. 3,873,911 and 3,909,708 has many advantages. For example, dynamic conductance testing utilizes electronic apparatus that is small and lightweight, draws very little current, produces virtually no "sparking" when connected or disconnected, does not appreciably discharge or polarize the battery, and yields very accurate,l reproducible, test results.
Two electronic battery tester embodiments are disclosed in U.S. Pat. No. 3,873,911; each of which accurately determines a battery's dynamic conductance and provides the operator with a numerical reading that is directly proportional to this quantity. The first embodiment comprises a bridge circuit that is brought to balance by the operator to obtain the numerical reading. The preferred second embodiment provides the operator with a direct readout that may be displayed numerically on a digital or analog meter. The operating principles of the preferred, direct-reading, second embodiment of the invention taught in U.S. Pat. No. 3,873,911 are based upon the theory of high-gain feedback amplifiers.
U.S. Pat. No. 3,909,708 likewise discloses two electronic battery tester embodiments. However, from the operator's point of view, their operation more closely resembles the operation of a traditional load-test apparatus than does operation of either of the numerical-reading embodiments disclosed in U.S. Pat. No. 3,873,911. Rather than obtaining a numerical measurement, the operator makes preliminary adjustments to knobs on the panel of the apparatus; setting them to the electrical rating and temperature of the battery undergoing test. The disclosed apparatus then employes small-signal measurements of dynamic conductance to simply ascertain whether or not the battery is capable of delivering an amount of power appropriate to the battery's rating and temperature. Accordingly, the two embodiments disclosed in U.S. Pat. No. 3,909,708 provide simple "pass-fail" battery condition information, just as does conventional load test apparatus. However, they accomplish this result without drawing a large current from the battery and are therefore not subject to the serious disadvantages of a load test. Just as with the second embodiment disclosed in the earlier patent, the operating principles of the second, preferred, embodiment disclosed in U.S. Pat. No. 3,909,708 are based upon the theory of high-gain feedback amplifiers.
Both preferred embodiments of electronic battery testing devices, the second embodiment disclosed in U.S. Pat. No. 3,873,911 and the second embodiment disclosed in U.S. Pat. No. 3,909,708, are based upon feedback amplifier principles. The original implementations of these electronic battery testing devices both utilized contemporary solid state device technology. Such technology was, however, limited to only discrete devices such as bipolar transistors and diodes, and small-scale integrated (SSI) circuit version of single-element monolithic operational amplifiers.
Great advances have been made in solid-state integrated circuit (IC) technology during recent years. In particular, high preformance complementary metal-oxide-semiconductor (CMOS) and bipolar medium-scale integrated (MSI) circuits, such as dual and quad operational amplifiers, have become abundantly available at very low prices. Therefore, important advantages, of both technical and economic natures, solid-state device technology in the electronic battery testing art.
Unfortunately, a number of design considerations preclude the simple introduction of the newer IC technology into the feedback-amplifier type of electronic battery tester circuitry disclosed in U.S. Pat. Nos. 3,873,911 and 3,909,708. Foremost among these considerations are the various problems imposed by the fact that the commercially available CMOS and bipolar MSI ICs do not provide separate pin-outs for supplying power to the individual elements on the chip. However, the original discrete-element feedback amplifier designs relied heavily upon the availability of such separate power connections. In particular, the original designs required separate connections for supplying power to different active devices in order to implement "four-point probe" architecture and thereby eliminate the spurious resistance of the connecting leads and battery contacts from the measurements; in order to realize a precisely-leveled oscillator voltage and thereby obtain increased measurement accuracy; and in order to implement synchronous detection of the amplified oscillator signal and thereby supress measurement errors caused by aspurious pickup of hum and noise. Accordingly, major changes in the basic design of the electronic battery tester embodiments would be required before one could realize any of the potential technical and economic benefits associated with the newer, more efficient and more cost-effective, IC technology.